Harden workflow actions for inconsistent workflows

in translate5 it is possible to create inconsistent workflows: For example reviewer and translators working in parallel.

The workflow actions should be hardened for this scenario.

This means:

• next workflow steps should only receive an email, that it is their turn, if they are in state waiting.
• If next workflow steps are finished, they should be skipped in the workflow chain and not opened again.
• If next workflow steps are open, the workflow step should simply change to their step and no mail notifcation to them should be send, because they are working anyway already.